Surface cracks are particularly dangerous faults in workpieces which are continuously stressed. Accordingly, components concerned with safety in the automobile industry must be non-destructively tested for freedom from cracks before they are installed. The magnetic powder method or the equally known dye penetration method is advantageously used to this end. In both methods the cracks, which cannot initially be recognized by the human eye, are displayed on the workpiece surface in a high-contrast and broadened form so that a tester can reliably recognize the indications.
Hitherto, such crack indications could be evaluated in practical industrial operation only by visual observation of the entire workpiece surface on the part of the tester. Such visual evaluation represents monotonous work, more particularly when testing mass-production components, and imposes a considerable stress on the eyes of the testing personnel. This has a detrimental effect on one's ability to concentrate. Moreover, subjective influences can hardly be excluded in visual evaluation. Reliable documentation of freedom from defects in industrial practice could therefore not be achieved in crack testing by means of the magnetic powder or dye penetration methods.
There is therefore an urgent need for facilities which are practicable in industrial terms to automate the operation of recognizing and evaluating the indications obtained in the non-destructive testing of workpieces for surface cracks.
According to the prior art, any departure from the magnetic powder method or the dye penetration method towards non-destructive materials testing methods which can be automated, for example ultrasonics, eddy current or stray flux measurements, is possible only if the specimens have a geometrical simple shape, testing can be confined to specific zones which are particularly endangered or if the defect direction can be predicted. Furthermore, the ability to recognize defects with this automatic method depends on surface texture so that by contrast to magnetic powder testing it is not possible to detect cracks whose depth does not substantially exceed the surface texture.
To detect hairline cracks in complicated workpieces with a cast or forged surface there are at present hardly any means other than the application of the magnetic powder method or the dye penetration method which necessitates visual evaluation of the indications thus obtained.
Efforts were made to achieve an improvement in the testing reliability by the use of indicating means which provide the greatest possible contrast of the crack indication on the workpiece surface, for example by black indicating means on a metallically bright surface, black or red means on a brightly dyed surface or by fluorescent indicating means which are brightly illuminated when observed in ultraviolet light, while the remaining surface remains dark.
Attempts have also been made to utilize such contrast, which increases optical recognizability, as the basis for automatic evaluation. For example, the illumination brightness of fluorescent indications was measured by means of photomultipliers. If the entire workpiece surface is covered in one pass, reliable recognition of crack indication is possible only if the remaining surface is kept free of fluorescent testing media. This procedure encounters difficulties in the case of rough surfaces; these retain a fine film of testing medium particles and, depending on the concentration of the testing medium fluid, results in a background brightness of greater or lesser intensity against which the additional brightness of a crack indication provides only a slight change.
As an alternative, there remained the restriction of the field of evaluation. Traversing over the workpiece surface made necessary thereby is time-consuming. It therefore also does not offer any solution to the problem because, dependent on the surface structure of the specimen or impurities in the testing medium fluid, it is possible for surface accumulations of testing medium to occur which in places can reach the brightness of the crack indication. These so-called apparent defects are however also evaluated by an observer. FIG. 1 shows a portion of a workpiece surface with a crack indication R and an apparent defect F.
It has already been proposed to utilize the different structure of crack indications and apparent defects and to use such difference as the basis for automating the evaluation of such indication. The different structure of the indication can be defined in terms of crack indications on the surface always having an elongated extent in one direction and being substantially narrower in the dimension at right angles thereto. To utilize this difference it was proposed to record the entire image and to store it electronically. The electronically stored image would then be tested by an electronic data processing plant to determine whether or not existing indications were line-shaped. The industrial use of this proposed method has not yet materialized due to the substantial effort required for storage and because of the very costly data processing plant.
It is the object of the present invention to provide means for automatically evaluating optical indications of cracks without such an effort but in addition to ensure that fault signals are delivered only by indications which are due to a surface crack and do not represent an apparent defect.
The method according to the invention is characterized in that the surface under observation is scanned by the light-sensitive device line by line, the width of each of which--either as individual line or as a group of adjacent lines--corresponds to the maximum optical display width from which evaluation is to proceed and that the bright-dark signals thus obtained from three lines or line groups, adjoining each other or separated from each other by an intermediate line and scanned in direct succession or simultaneously, where appropriate after storage of the signals associated with the first scanned two lines or line groups, are compared and from the signal of the middle line or line groups and the signal of the two other lines or line groups there is formed a difference which is evaluated to generate an error evaluation signal if a minimum value is exceeded.
Scanning of the observed surface can be performed in one operation by successive scanning of adjoining lines or groups of lines, where appropriate, accompanied by simultaneous scanning of every three adjoining lines or by the so-called line interlace method in which scanning is performed in two passes, offset by one line with respect to each other and by the omission of one line so that crack indications on the boundary between two lines or groups of lines can be evaluated with greater reliability. A three-gun image recording device, comprising three vidicons or a "chopped" vidicon can be used for scanning. With the present state of the art of image recording tube technology it appears to be more efficient to project the observed surface on the screen of a single-gun image recording apparatus and to successively scan the three lines or groups of lines on which evaluation is based by means of one electron beam and by storing the bright-dark signals of the two first-scanned lines or groups of lines. The surface to be tested can also be scanned through line by line illumination with one or more light beams or laser beams.
To evaluate indications of any random direction the surface image can be mechanically, optically or electronically rotated during one testing cycle.
To specify a minimum length of optical indication as the limit for the commencement of evaluation, it is possible to arrange for the bright signals to be evaluated only onwards from a specified minimum appearance time.
Surface regions with defect indications can be blocked out mechanically, optically or electronically.